This invention relates to devices to be used for the production of monocrystalline ingots by pulling from a molten bath, particularly the production of monocrystalline silicon ingots using the Czochralski process. The invention specifically relates to the crucible holders (or xe2x80x9csusceptorsxe2x80x9d) made of carbon materials used in these devices.
The production of microelectronic silicon-based components such as memories and microprocessors uses, as raw material, slices of silicon ingots (commonly called xe2x80x9cwafersxe2x80x9d) cut from monocrystalline silicon ingots. The most frequently used process for the production of these monocrystalline ingots, known as the xe2x80x9cCzochralski processxe2x80x9d, or more simply the xe2x80x9cCZ processxe2x80x9d, essentially comprises pulling a solid ingot from a molten silicon bath.
Many industrial developments have been carried out on this process in order to satisfy the continuously increasing demand for wafers and to improve the perfection of the crystals obtained, which is critical for the efficient manufacture of microelectronic components. CZ pulling equipment according to the state of the art is relatively standardized and comprises essentially a sealed metallic containment (or xe2x80x9covenxe2x80x9d), thermal insulation means, heating means, a crucible, and a crucible holder (or xe2x80x9csusceptorxe2x80x9d). During pulling operations, the crucible contains extremely pure silicon which is molten and then xe2x80x9cpulledxe2x80x9d as a monocrystal. The crucible is usually made of quartz, so that the silicon does not come into contact with a material containing elements other than silicon and oxygen, which prevents pollution of the molten silicon bath by impurities that would deteriorate the monocrystal quality.
Since the melting temperature of silicon is very high (1430xc2x0 C.), the quartz crucible must be supported by a crucible holder, since at this temperature quartz no longer has sufficient mechanical strength to support its own weight, and even less the weight of the liquid silicon contained in it. The main function of the crucible holder that forms a sort of external envelope, is to support the crucible while the silicon crystal is being pulled.
For several technical and economic reasons, the crucible holder is usually graphite based. Firstly, the mechanical properties of graphite remain high at the working temperatures normally used. Secondly, it is possible to obtain a graphite that does not contain any metallic impurities that could pollute the molten silicon bath by diffusion. Finally, the cost of the graphite material is competitive.
However, the use of a graphite based crucible holder does create some technical problems. Firstly, since the coefficient of expansion of graphites used is systematically very much greater than the coefficient of expansion of quartz within the range of temperatures at which the mechanical properties of quartz remain high (namely typically from 3 to 6xc3x9710xe2x88x926 Kxe2x88x921 at up to about 1015xc2x0 C. for the most frequently used graphites and 0.5xc3x9710xe2x88x926 Kxe2x88x921 for quartz), the use of monolithic crucible holders generates high mechanical stresses in the crucible holder and in the crucible during a temperature cycle and frequently destroys one or the other. More precisely, when the temperature of the crucible and the crucible holder is increasing, the crucible expands less than the crucible holder, thus creating a space between these two components; when the temperature reaches values at which the quartz can creep, the crucible deforms and fills the space between the two elements; when the assembly cools (normally after the pulling operation) the graphite contracts, compresses the crucible and deforms it as long as it is in the paste phase (in other words as long as the temperature exceeds about 1015xc2x0 C.); subsequently during cooling, the crucible becomes very solid and resists contraction of the crucible holder, causing the development of compression stresses on the crucible and tension stresses on the crucible holder. One way of solving this problem that has been known for more than 15 years is to use crucible holders composed of separate parts that may be connected together or assembled in an approximately butt jointed manner. During use, the parts are kept more or less in contact by means of an appropriate support plate that allows slight relative displacement of the parts with respect to each other during pulling cycles. These relative displacements, which are essentially either a relative movement towards or away from each other, absorb expansions of the crucible and the crucible holder. Typically, there are at least three parts.
Furthermore, the high temperatures reached during pulling operations cause chemical reactions in contact areas between the graphite crucible holder and the quartz crucible. In particular, it is observed that the crucible holder is consumed and silicon carbide is produced on the surface of the graphite. Progressive consumption of the crucible holder limits its life and creates deposit of silicon carbide particles that do not adhere well and are released in the form of dust that is harmful to instruments and the quality of silicon crystals. This wear of the crucible holders obliges users to replace them every 30 to 50 pulling cycles.
Therefore, the applicant decided to look for economic and industrially applicable solutions that could prolong the life of crucible holders made of a carbon material, particularly graphite based crucible holders, while maintaining their working properties.
The purpose of the invention is a crucible holder made of a carbon material, particularly graphite based, designed for crystal pulling operations and including at least two distinct and complementary parts that can be joined together by xe2x80x9cjunctionxe2x80x9d surfaces, and characterized in that specific portions of the said junction surfaces overlap when the said parts are brought together to form the crucible holder, thus forming xe2x80x9coverlapxe2x80x9d areas.
The applicant had the idea that the consumption of the crucible and the production of silicon carbide at the surface of the carbon material could be largely related to the development of openings or interstices between the parts of crucibles according to prior art when they are being used, the said openings or interstices could encourage thermochemical reactions between the silica in the crucible and the carbon in the crucible holder. The applicant put forward the theory that a large proportion of the degradation mechanisms could be assigned to thermochemical reactions that involve the production of carbon oxides by reaction of silica with carbon (such as the SiO2+Cxe2x86x92SiO+CO and SiO+2Cxe2x86x92SiC+CO reactions).
The applicant performed tests to verify that the presence of overlap surfaces between parts of the crucible results in a net reduction in the consumption of crucibles and the production of silicon carbide, despite the relative displacements that occur during pulling cycles. The applicant considers that the overlap makes the junction surfaces more impermeable and increases the length of gas diffusion path. The overlap can limit, and possibly block, gaseous diffusion between the inner and outer surfaces of the crucible holder, such that the thermochemical reactions causing degradation of the crucible holder that introduce gaseous phases become negligible. The applicant also considers that the parts do not separate in practice, or separate very little during pulling cycles, so that the impermeability resulting from the overlap can be maintained.
The invention will be better understood after looking at the figures and reading the following detailed description.